A high power permanent magnet synchronous motor, in general, uses numbers of teeth of a stator and employs a distributed winding method so that the composite magnetomotive force of this motor can approximately form a sine wave. Permanent magnets of the rotor of this synchronous motor employs magnets made of rare-earth material featuring a high density of magnetic flux as well as large withstanding forces against demagnetization. Further, a sensor detects a rotational phase of the rotor so that the current phase can be controlled responsive to a rotor position.
However, the distributed winding method requires complicated winding processes, and this lowers winding-efficiency. The rare earth magnet and the sensor detecting the rotational phase are expensive, and these elements boost the cost of this motor.
An inexpensive permanent magnet synchronous motor is thus developed as shown in FIG. 17(a) in order to overcome the problems discussed above. Stator 21 is formed by cores 22 (refer to FIG. 17(b)) divided corresponding to respective teeth. Teeth 26 of divided cores 22 are wound with insulating paper 28, and coils are wound on top of that, thereby forming concentrated winding coils 23. Divided cores 22 with the concentrated windings are incorporated into a ring and fixed by welding, caulking or laser-beam-welding to form the stator having the concentrated windings. Permanent magnets 25 of rotor 24 are made of inexpensive-ferrite magnet. Regarding the current-phase control, a zero-cross point of an inductive voltage produced by a neutral coil which allows no driving current to run through—is detected so that 120 excitation can be executed by rectangular waveforms.
In this permanent magnet synchronous motor, 3n (n=a natural number) pieces of teeth of stator 21 are arranged at equal intervals and the teeth are coupled with each other to form three phases through “Y” letter connecting method. Permanent magnets with 2n (n=a natural number) poles are arranged to face stator 21. As such, it is preferable to prepare 2n poles of permanent magnets for 3n pieces of teeth in the permanent magnet synchronous motor.
In the example shown in FIG. 17, a number of poles of rotor 24 is 8 poles (2n, n=4), a number of stator teeth is 12 (3n, n=4). Respective teeth are wound with coils u1, v1, w1, u2, . . . v4, w4 sequentially. Each coil is connected in series as shown in FIG. 18(a) or in parallel as in FIG. 18(b) to form phases U, V and W.
Meanwhile, in an ordinary permanent magnet synchronous motor, the following relation is established so that leakage flux between each tooth can be reduced: La>approx. 2 Lg, (refer to FIG. 19) where La is a clearance between teeth 26 and 26, and Lg is an air gap between stator 21 and rotor 24.
Permanent magnets 25 have even thickness from the first end to the second end in a circumference direction, and magnets 25 are arranged so that each end thereof faces another adjacent end. However, if this structure is applied to the inexpensive permanent magnet synchronous motor as discussed above, the permanent magnets encounter local demagnetization due to the following reason, whereby a desirable output cannot be produced by the motor.
Since the motor employs the concentrated winding method, a tooth bears a different pole from that of its adjacent tooth, thereby increasing inductance. This situation allows the rotor to be subject to demagnetization. In particular, when the motor is in a sensor-less operation, the permanent magnets of the rotor tend to be demagnetized at starting or at out-of-sync condition. In other words, as shown in FIG. 20, stator coil 23 produces a pole counteracting a pole of permanent magnet 25 of rotor 24, and parts of magnetic field produced by coil 23 invade permanent magnets 25 as demagnetizing magnetic field 27. When permanent magnets 25 are made of ferrite magnet, demagnetizing magnetic field 27 renders magnets 25 into break down condition. As a result, magnets 25 are demagnetized.
Numbers of motors with concentrated windings have been available in the market. However, a clearance between teeth is so narrow that the permanent magnets are subject to demagnetization when the polarities of adjacent teeth are opposite each other. When the permanent magnet made of ferrite having small coercive force is used, the withstanding force against demagnetization becomes poor. When the motor is in the sensor-less operation in particular a reverse magnetic field is applied to the permanent magnets at starting or out-of-sync condition, thereby demagnetizing the permanent magnets with ease.
The present invention addresses the problems discussed above, and aims to provide a permanent magnet synchronous motor, in which the concentrated winding method is employed and yet the withstanding force of the permanent magnets against demagnetization is enhanced.